8.2.4.2 Optimizing UPDATE Statements

An update statement is optimized like a
SELECT query with the
additional overhead of a write. The speed of the write depends
on the amount of data being updated and the number of indexes
that are updated. Indexes that are not changed do not get
updated.

Another way to get fast updates is to delay updates and then
do many updates in a row later. Performing multiple updates
together is much quicker than doing one at a time if you lock
the table.